Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to Fresnel reflectors, and more particularly to monolithic non-planar Fresnel reflector arrays, and mold patterns and mold structures for manufacturing the arrays which eliminate or substantially reduce negative draft during molding.
2. Description of the Background Art
Reflectors are often utilized within various detector and illumination assemblies to focus incident radiation onto a detector (sensor), or to direct radiation emitted from an illumination source toward specific directions. For example, a Fresnel reflector is capable of focusing radiation from a dispersed target zone onto a focal zone, and for directing radiation emanating from an illumination source located near the focal zone to a dispersed target zone. The focal zone comprises a volume that is at, or generally near, the focal point of the reflector. Fresnel reflectors are often utilized to reduce the size and cost of a reflector within a given application. Each Fresnel reflector comprises a series of joined reflector segments configured as rings that may be of a similar shape and/or angle to the sections within an equivalent curved reflector, such as spherical, aspheric, parabolic, or hyperbolic. The rings in a simple Fresnel reflector, however, are placed concentrically on a single plane with steps leading from one ring to the next. Fresnel reflectors may be employed for either focusing radiation, or dispersing radiation, such as radiation found within the visible, infrared, or ultraviolet spectrums.
Motion sensors utilized in alarm systems make extended use of these Fresnel reflectors for focusing radiation, such as at visible, near-infrared, and infrared wavelengths toward a suitable detector. Typically, the surface of the reflector is configured with a mirrored surface that reflects the optical wavelengths toward the detector. In some systems, a non-planar array of Fresnel reflector segments is utilized to collect radiation from a detection area which covers a wide angular spread. The individual Fresnel segments within the non-planar array are aligned adjacent one another at angles to form a generally curved shape. Often complex reflectors are created utilizing multiple tiers of these non-planar arrays, whereas the combination of tiers are typically configured with a single focal zone.
Illumination systems, such as those utilized in the law enforcement/rescue industry and illumination systems utilized in the commercial lighting industry make extended use of Fresnel reflectors for dispersing light which emanates from an illumination source, such as an incandescent lamp, a fluorescent lamp, arc lamp, strobe, an LED (visible or infra-red), ultra-violet lamp, infrared lamp, and similar illumination sources and combinations thereof. Use of Fresnel reflectors for directing illumination is not limited to single illumination sources, as multiple illumination sources may be combined, such as LEDs generating different colors, and placed sufficiently close to the focal point of the reflector system to benefit from being directed by the reflector. Light beacons can utilize Fresnel reflectors in a number of ways. For example, reflectors being utilized for general illumination purposes can be constructed with fixed reflectors to distribute light from an illumination source. For example, the mirrored Fresnel reflector can be utilized to create a regular pattern of bright spots such as radial patterns of bright bars and star effects, and so forth. Rotating emergency beacons can be designed, for instance using a halogen light around which a mirrored Fresnel reflector assembly is rotated.
A conventional single-tier non-planar Fresnel array is shown in FIG. 1 having segments which direct radiation to, or from, a focal point. The figure illustrates radiation being received from a radiation source in the target zone and reflected from the reflector toward a pyro-electric detector. It will be appreciated that Fresnel arrays for the visible portion of the electromagnetic spectrum are typically manufactured from injection molded plastic to which a mirrored surface is applied, and that each Fresnel segment generally comprises an on-axis center section of a Fresnel reflector.
FIG. 1 depicts a conventional, single-tier, non-planar Fresnel reflector array 10 comprising the following series of joined Fresnel segments: first segment 12, second segment 14, third segment 16, fourth segment 18, and fifth segment 20. Each of the Fresnel segments within the array has an optical axis centered within the center ring of the Fresnel reflector. Light paths 22, 24, 26, 28, 30, are shown extending between the center of each segment 12, 14, 16, 18, 20, respectively, to a detector 32 (shown in phantom) having a focal point FP. The dashed line of the light paths represents the center of the light which reflects from reflector 10. Detector 32 is preferably positioned within the focal zone of the Fresnel reflector which is sufficiently near focal point FP to provide the desired pattern of reflection. In the figure, Fresnel segments 12, 14, 16, 18, and 20, are generally arranged so that the focal point of each Fresnel segment is aligned with focal point FP of detector 32. Radiation is exemplified as being received from a central portion 34 of a target zone 36, which may be referred to as a detection area, or detection zone, within an alarm system. It will be appreciated that the light path from the target zones to reflector 10 is shown overlapping the light path from reflector 10 to detector 32. It should also be appreciated that descriptions reciting the focusing of radiation onto a sensor or detector at the target zone or focal point, are generally applicable in the reverse direction for directing illumination away from an illumination source at the target zone, or focal point, and distributing the illumination according to the reflection pattern of the Fresnel lens. An angular offset, perpendicular to the plane of the illustrations, allows the radiation to pass underneath the detector for receipt by the segments of the non-planar Fresnel reflector array and subsequent reflection onto the detector. The non-planar Fresnel array is shown configured to receive radiation for a given arc of the target zone, wherein the inner three segments receive radiation from a first angular spread 34, such as forty five degrees (45xc2x0), and the exterior two segments extend the angle to a second angular spread 36, such as ninety degrees (90xc2x0). It will be appreciated that an alarm system utilizing the illustrated non-planar Fresnel reflector may have greater sensitivity to radiation being received from the first angular spread 34, due to increased levels of radiation being coupled from the reflector to the detector. Furthermore, the Fresnel segments extending from center segment 16 are retained at progressively increasing angles in relation to center segment 16 in order to retain focus on focal point FP of detector 32. A first tilt angle 38 is illustrated as 22.50 degrees, and a second tilt angle 40 is shown at an angle of 45.00 degrees. Joints 42, 44, 46, 48, between the Fresnel segments provide retention of the segments at the desired angle, although the segments may be mounted to a backing assembly to retain proximal retention of the adjacent segments. The Fresnel reflector tier of the figure is molded as five separate reflectors which are then joined to one another, or otherwise retained proximal to one another. It should be appreciated that a single mold incorporating the five segments of Fresnel array 10 as a monolithic structure would be subject to inclusive draft within the mold, irrespective of the chosen mold release pull direction. Given a particular pull direction 50, a monolithic molded structure of non-planar Fresnel array 10 would contain inclusions within region 52 of each Fresnel segment that is non-orthogonal to the pull direction. The inclusive draft regions 52 being depicted between opposing brace symbols which bracket the regions subject to inclusive draft. In relation to pull direction 50, regions 52 contain portions of the mold subject to inclusive draft, also referred to as negative draft, or undercut. Extraction of the finished part from its corresponding mold, therefore, is prevented by the interlocking of the injected plastic with the mold. Forced removal of the monolithic array is typically not an option, as even in the best cases it would substantially increase breakage and produce damaged reflector arrays. The undesired inclusive regions 52 surround each reflective ring 54 in a portion of step 56 toward a succeeding ring. Each step within a Fresnel reflector is typically formed with a surface that is directed orthogonal to the plane of the Fresnel reflector, wherein a tilt thereof causes the steps on one half of the Fresnel segment to contain inclusions.
The problems associated with molding inclusions have led to the use of inefficient methods for creating multi-element Fresnel reflectors comprising joined Fresnel segments. For example, the Fresnel segments may be molded as separate sections and joined, or the mold may be carefully modified, such as by filling in portions of the Fresnel steps which are subject to undercutting. The use of separate Fresnel reflector sections is not cost effective as it requires elements to be separately manufactured, positioned, aligned, and fastened to form a reflector array. The process, however, for changing the angle or profile of the steps between the rings is tedious and it can result in a loss of reflective area and distortions.
Therefore, a need exists to simplify the creation of non-inclusive mold patterns for use in the manufacture of non-planar Fresnel reflector arrays. The present invention satisfies that need as well as others and overcomes the deficiencies of previously developed solutions.
The present invention provides a monolithic non-planar Fresnel reflector array mold pattern, and method of mold fabrication thereof, which simplifies molding and essentially eliminates inclusive draft. The Fresnel reflector array mold pattern of the present invention comprises a plurality of Fresnel segments that are arranged edge-to-edge at a relative angular offset to form a Fresnel reflector array, in which at least one of the Fresnel reflector segments has a focal point along a path that is angularly offset from the optical axis of the Fresnel segment. The Fresnel segments having focal points which are angularly offset from the optical axis are referred to herein as xe2x80x9coff-axisxe2x80x9d segments. It will be appreciated that an off-axis segment may be created having an offset in either axis, or combination thereof, within the plane of Fresnel segment. The use of off-axis segments within the Fresnel reflector array can eliminate inclusive drafts that would otherwise arise when molding a non-planar Fresnel reflector array having a plurality of Fresnel segments.
By way of example, the non-planar Fresnel reflector arrays according to the present invention are typically utilized within motion detectors associated with devices such as alarm systems, and as illumination reflectors such as found in emergency beacons or general-purpose residential illumination. It should be appreciated, however, that the use of the Fresnel arrays manufactured from mold patterns according to the present invention are not limited to motion detectors and illumination beacons, but are applicable to any device in which non-planar Fresnel arrays may be utilized. Non-planar Fresnel reflector mold patterns according to the present invention eliminate the inclusive draft exhibited by previously developed non-planar array mold patterns, while reducing the necessary angle for achieving focus on the detector which results in a reduction of the overall height of the non-planar array structure. The present invention utilizes different optical relationships for creating and orienting the constituent Fresnel segments within the mold pattern of the array. Mold patterns according to the present invention utilize Fresnel segments created from off-axis Fresnel regions, such as removed from a representative Fresnel master, which exhibit a skewed focal point that is corrected by changing the angular relationship between the segments. As a result, the steps within the off-axis segments are oriented in relation to the pull direction to eliminate inclusive draft. The resultant non-planar Fresnel reflector mold patterns focus light toward, or from, one or more focal points, or target zones. The Fresnel reflector arrays of the present invention provide a number of advantages over conventional Fresnel arrays including a reduction in the necessary array depth and the elimination of the inclusive draft so that even complex multi-tier arrays may be readily manufactured.
The teachings of the present invention are applicable to any form of segmented non-planar Fresnel reflector array. The off-axis Fresnel segments utilized according to the present invention may be from any form of Fresnel reflector including those which are configured with uniform groove depth, uniform groove width, variable groove width, or variable groove depth, or any combination thereof.
The term mold pattern as used herein, comprises a pattern that is created of suitable material and mold polarity, either positive or negative, that may be utilized with any number of interceding steps between the making of an original pattern and the resultant tool that is utilized during the manufacture of the final non-planar Fresnel array reflector parts. For example, a negative mold pattern can be created from a tool grade material, wherein after assembly it can be directly utilized as the tool. Generally, however, the mold pattern is created from easily workable materials and utilized for molding the next in a succession of intermediate molds before the creation of the final tool used in manufacturing. Often electroforming is employed within the process of creating molds from mold patterns. It will be appreciated that the front surface of the manufacturing tool from which the final parts are molded is substantially identical to the original mold pattern which is formed in a polarity suited to the sequence of steps used in progressing from a mold pattern to a final tool, or manufacturing mold.
By way of example, a method is described for creating a mold pattern for a non-planar Fresnel reflector array following the generalized steps of: (1) determining the shape and xe2x80x9ccutxe2x80x9d of a segment as based on final position within a populated reflector array wherein segments that will be oriented non-orthogonally to the pull direction of the Fresnel array are cut from off-center portions of the representative Fresnel master; (2) severing segments from a Fresnel master that is constructed of any material appropriate to the succeeding steps toward the creation of the final manufacturing mold wherein the interior edges of the severed segments are configured for attachment to adjacent Fresnel segments within the array; (3) joining the segments into a three dimensional mold pattern. The center ring, or curved facet, portion of each off-axis Fresnel reflector segment is therefore bisected by an edge of the off-axis Fresnel reflector segment, that edge being oriented toward the center of the array which preferably contains an on-axis segment. It will be appreciated that the side of the Fresnel reflector segment associated with the edge that bisects a portion of the center ring preferably contains the largest percentage of the center ring.
It will be appreciated that prior to assembling the array, the Fresnel segments may be joined to an intermediate element, such as an angled block, or joined to a structure configured for retaining the segments in an aligned position. The segments may be aligned in a number of ways within the array, such as positioning each segment at a radial distance, from reflective face to focus, which has a predetermined relationship; such as at a fixed radial distance, or a distance characterized by a specific mathematical function, such as parabolic. Another example method provides for aligning the reflector segments in response to the physical joints that are created between each segment, wherein joints are created having an interface with a particular set of characteristics; for example, orienting the faces of the segments such that the joint between each set of elements is substantially flush and continuous.
In the process of severing a segment from a Fresnel master, the edges of the segment are cut, or shaped, to facilitate joining with adjacent segments. Generally, it is desirable to eliminate gaps in the reflector surface to maximize surface area and strength, however, this is not a precondition of the present invention. To eliminate the gaps, the edges of adjacent segments are cut to join to one another with a maximum amount of mating surface area when the segments are correctly positioned within the array. The edges of the segments may be cut in various ways to maximize the joint area. For example, xe2x80x9cmiterxe2x80x9d style joints may be utilized wherein the edge angle on adjacent segments is complementary, or parallel cuts may be made facing a particular direction in relation to the final non-planar array. It is preferred that the segments be cut with parallel severing lines such that the surfaces of the resultant joint between adjacent segments is parallel to the direction of pull to thereby allow for changes in surface alignment from one segment to the next without altering the overall width of the Fresnel array or contributing inclusions when misalignment at the surface occurs.
The process of severing segments from a Fresnel master is described in terms of separating, or cutting, a section (typically rectangular) from a Fresnel blank, or master, of the appropriate material and polarity. A variety of methods may be utilized for severing a segment from the master, such as saw cutting, laser cutting, EDM cutting (electro-discharge machining), fluid cutting, and so forth. Although cutting is a preferred method, it should be appreciated that segments which correspond with, or represent, a given portion of a Fresnel master may be fabricated in a number of additional ways. For example, subtractive methods such as grinding or etching may be utilized to remove material within the blank to arrive at the final Fresnel segment. In addition, additive processes may be utilized to create a segment formed into the correct size, shape, edge configuration, and pattern of rings and steps that are equivalent to that of a segment being severed from a Fresnel blank, or master. In addition, articles of any shape may be formed within solids fabrication systems wherein Fresnel segments may be created having a size, shape, and ring pattern homologous with a segment being cut from a Fresnel master. It should be appreciated that a Fresnel master is typically a Fresnel reflector pattern in the form of a blank Fresnel reflector, typically circular. A non-planar Fresnel array mold pattern may be created by assembling portions from a series of Fresnel masters, or equivalent sections thereof. It will be appreciated that Fresnel portions that are fabricated by methods other than severing from a Fresnel master will still contain a pattern which corresponds to a given representative Fresnel master.
An object of the invention is to provide a monolithic non-planar Fresnel reflector array mold pattern which is free of molding inclusions.
Another object of the invention is to provide a monolithic non-planar Fresnel reflector array mold pattern in which the step and ring geometry of the Fresnel segments do not require modifications to eliminate inclusions.
Another object of the invention is to provide a method of making Fresnel array mold patterns which are not subject to mold inclusions.
Another object of the invention is to provide a mold pattern utilizing segments from Fresnel reflector masters without regard to reflector type, which by way of example can include uniform groove depth, uniform groove width, variable groove width, variable groove depth, combinations thereof, as well as other forms of Fresnel reflectors.
Another object of the invention is to provide a mold pattern for a non-planar Fresnel reflector array wherein the structure of the joint between successive segments is not subject to inclusions.
Another object of the invention is to provide a method of creating mold patterns for non-planar Fresnel reflector arrays containing one or more non-planar (curving) tiers of reflector arrays.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.